Method of Forming a Composite Conductive Film

ABSTRACT

A method of fabricating a composite conductive film is provided. The method includes providing, as a matrix, a layer of cross-linkable polymer, where the cross-linkable polymer is in a non-cross-linked state. The method further includes introducing inorganic nanowires upon a surface of the layer of cross-linkable polymer. The inorganic nanowires are, in isolated form, characterized by a first conductivity stability temperature. The method further includes embedding at least some of the inorganic nanowires into the layer of cross-linkable polymer to form an inorganic mesh, thereby forming the composite conductive film. The method further includes cross-linking the polymer within a surface portion of the composite conductive film. Cross-linking the polymer within the surface portion of the composite conductive film results in the surface portion having a second conductivity stability temperature that is greater than the first conductivity stability temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is divisional application of U.S. patent applicationSer. No. 14/486,775, entitled “Composite Conductive Films with EnhancedThermal Stability,” filed Sep. 15, 2014, which is a continuation-in-partof U.S. patent application Ser. No. 14/205,001, filed Mar. 11, 2014,which, in turn, claims priority to U.S. Provisional Patent ApplicationNo. 61/792,924, filed Mar. 15, 2013, entitled “Photoactive TransparentConductive Films Patterned Via Abbreviated Photolithography,” and U.S.Provisional Patent Application No. 61/949,727, filed Mar. 7, 2014,entitled “Photoactive Transparent Conductive Films,” both of which arehereby incorporated by reference in their entirety.

This application is also related to U.S. patent application Ser. No.13/194,134, filed Jul. 29, 2011, entitled “Conductive Films”; and U.S.patent application Ser. No. 14/486,815, filed Sep. 15, 2014, entitled“Composite Conductive Films with Enhanced Surface Hardness,” now U.S.Pat. No. 9,491,853; each of which is hereby incorporated by reference inits entirety.

TECHNICAL FIELD

The disclosed embodiments relate generally to transparent conductivefilms and, more particularly, to composite transparent conductive filmsthat are imparted with the properties of a matrix material employedtherein (e.g., a property such as photoactivity, hardness, and/orthermal stability).

BACKGROUND

The embodiments described below relate to conductive surfaces and/orfilms with enhanced hardness properties and/or temperature stability.

Hardness is a measure of a material's ability to resist scratchingand/or indentation. Materials with harder surfaces are often better ableto withstand certain types of wear and damage, and thus are more durableand desirable in many applications. Conventional methods of enhancingthe surface hardness of a material include depositing a hard layer ontop of a softer surface. But when the surface needs to be accessible(e.g., because it has a certain property), covering the surface is notfeasible. For example, an electrically conducting surface cannot becovered if future access to the surface is needed. Thus, there is a needfor materials and/or films that have hard electrically conductingsurfaces.

Likewise, there are many applications in which it is desirable to haveinexpensive conductive films. Conventional conductive films, however,provide less than satisfactory temperature stability. One consequence ofpoor temperature stability is that the conductivity properties of suchfilms are often destroyed when the films get too hot.

SUMMARY

Disclosed are composite conductive films, methods of fabricatingcomposite conductive films, and devices that incorporate compositeconductive films. More particularly, the composite conductive filmsdescribed herein have a surface that is electrically conducting withenhanced hardness properties and/or temperature stability.

In particular, to address the aforementioned problems with conventionalcomposite conductive films, the disclosed embodiments provide acomposite conductive film with enhanced hardness properties. Thecomposite conductive film includes a layer of cross-linked polymerhaving a surface and an inorganic mesh comprising a plurality ofinorganic nanowires. The plurality of inorganic nanowires is embeddedthroughout at least a region of the layer of cross-linked polymer. Theregion is continuous from the surface of the layer of cross-linkedpolymer. The layer of cross-linked polymer and the inorganic mesh arearranged to form the composite conductive film. The composite conductivefilm has a pencil test hardness in a range of 2H to 9H.

In some embodiments, respective nanowires of the plurality of nanowiresof the inorganic mesh comprise a material having a conductivity between1×10⁵ S/cm and 1×10⁶ S/cm, or greater. In some embodiments, theinorganic nanowires comprise a metallic material.

In some embodiments, the plurality of nanowires has an average aspectratio of the nanowires between 10-1000.

In some embodiments, the layer of cross-linked polymer comprises one ofan acrylated polyurethane or an epoxy-based alkyl-amine.

In some embodiments, the composite conductive film is disposed on asubstrate. The substrate is one of a polyester substrate or aborosilicate glass substrate.

In some embodiments, the layer of cross-linked polymer comprises apolymer that undergoes cross-linking when exposed to one of ultraviolet(UV) radiation, heat treatment, or a chemical catalyst.

In some embodiments, the respective inorganic nanowires of the pluralityof inorganic nanowires are fused to form a substantially continuousnetwork over a continuous portion of the composite conductive film. Insome embodiments, the substantially continuous network of the inorganicnanowires is confined to the region of the layer of cross-linkablepolymer that is continuous from the surface, thereby forming asubstantially two-dimensional continuous network of the inorganicnanowires at the surface of the layer of cross-linkable polymer.

In some embodiments, the respective inorganic nanowires of the pluralityof inorganic nanowires are metallic. Further, the plurality of nanowiresis characterized by a density within the region that is above atwo-dimensional percolation threshold and less than a bulk percolationthreshold of the inorganic nanowires suspended throughout the layer ofcross-linkable polymer.

In some embodiments, the surface region has a thickness. The compositeconductive film is characterized by a first surface roughness that isless than a second surface roughness of a stand-alone film consisting ofthe same thickness of the inorganic nanowires. In some embodiments, thefirst surface roughness is about fifty percent less than the secondsurface roughness. In some embodiments, the thickness of the region isselected in accordance with a predetermined target conductivity value.

In some embodiments, the composite conductive film is bendable to abending radius of at least 0.5 millimeters (mm).

Further, to address the aforementioned problems with conventionalcomposite conductive films, the present disclosure provides a method offabricating a composite conductive film with enhanced surface hardness.The method includes providing, as a matrix, a layer of cross-linkablepolymer. The cross-linkable polymer is in a substantiallynoncross-linked state. The method further includes introducing aplurality of inorganic nanowires onto a surface of the layer ofcross-linkable polymer. The method further includes embedding at leastsome of the plurality of inorganic nanowires into the layer ofcross-linkable polymer to form an inorganic mesh within the layer ofcross-linkable polymer, thereby forming the composite conductive film.The method further includes cross-linking the cross-linkable polymerwithin at least a surface portion of the composite conductive film.Following the cross-linking, the cross-linkable polymer within at leastthe surface portion of the composite conductive film is in across-linked state. Further, cross-linking the cross-linkable polymerwithin at least the surface portion of the composite conductive filmresults in a surface pencil test hardness of the composite conductivefilm that is between 2H and 9H.

This method is, in some embodiments, used to produce, fabricate and/ormanufacture any of the composite conductive films described herein.

Further, to address the aforementioned problems with conventionalcomposite conductive films, the disclosed embodiments provide acomposite conductive film with enhanced temperature stability. Thecomposite conductive film includes a layer of cross-linked polymerhaving a surface and an inorganic mesh comprising a plurality ofnanowires of an inorganic material. The nanowires are, in isolated form,characterized by a first conductivity stability temperature. Theplurality of nanowires is embedded within at least a region of the layerof cross-linked polymer, where the region is continuous from the surfaceof the layer of cross-linked polymer. The layer of cross-linked polymerand the inorganic mesh are arranged to form the composite conductivefilm having a second conductivity stability temperature that is greaterthan the first conductivity stability temperature.

In some embodiments, the composite conductive film is characterized by asheet resistance that is substantially constant as a function oftemperature between room temperature and the second conductivitystability temperature.

In some embodiments, the composite conductive film is characterized by asheet resistance having a temperature coefficient that is less than0.002 (ΩK)⁻¹ between the first conductivity and the second conductivitystability temperature.

In some embodiments, the layer of cross-linked polymer comprises one ofa silicone material, a polyamide material, or a polyimide material.

In some embodiments, the layer of cross-linked polymer comprises apolymer that undergoes cross-linking when exposed to one of ultraviolet(UV) radiation, heat treatment, or a chemical catalyst.

In some embodiments, the respective nanowires of the plurality ofnanowires are fused to form a substantially continuous network over acontinuous portion of the composite conductive film. In someembodiments, the substantially continuous network of the nanowires isconfined to the region of the layer of cross-linked polymer that iscontinuous from the surface, thereby forming a substantiallytwo-dimensional continuous network of the inorganic nanowires at thesurface of the layer of cross-linked polymer.

In some embodiments, the inorganic nanowires comprise a metallicmaterial. In some embodiments, the respective inorganic nanowires of theplurality of inorganic nanowires are metallic. Furthermore, theplurality of nanowires is characterized by a density within the regionthat is above a two-dimensional percolation threshold and less than abulk percolation threshold of the inorganic nanowires suspendedthroughout the layer of cross-linkable polymer.

Further, to address the aforementioned problems with conventionalcomposite conductive films, the present disclosure provides a method offabricating a composite conductive film with enhanced surface hardness.The method includes providing, as a matrix, a layer of cross-linkablepolymer, wherein the cross-linkable polymer is in a non-cross-linkedstate. The method further includes introducing a plurality of inorganicnanowires upon a surface of the layer of cross-linkable polymer. Theinorganic nanowires are, in isolated form, characterized by a firstconductivity stability temperature. The method further includesembedding at least some of the plurality of inorganic nanowires into thelayer of cross-linkable polymer to form an inorganic mesh within thelayer of cross-linkable polymer, thereby forming the compositeconductive film. The method further includes cross-linking thecross-linkable polymer within at least a surface portion of thecomposite conductive film. Following the cross-linking, thecross-linkable polymer within at least the surface portion of thecomposite conductive film is in a cross-linked state. Cross-linking thecross-linkable polymer within at least the surface portion of thecomposite conductive film results in at least the surface portion of thecomposite conductive film having a second conductivity stabilitytemperature that is greater than the first conductivity stabilitytemperature.

This method is, in some embodiments, used to produce, fabricate and/ormanufacture any of the composite conductive films described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a perspective view of a device that includes atransparent conductive film, in accordance with some embodiments.

FIG. 1B illustrates a perspective view of a device that includes acomposite conductive film with enhanced surface properties (e.g.,hardness or temperature stability), in accordance with some embodiments.

FIGS. 2A-2E illustrate a method of fabricating (e.g., manufacturing) acomposite conductive film, in accordance with some embodiments.

FIGS. 3A-3C illustrate a method of patterning a photoactive transparentconductive film using an abbreviated lithographic process, in accordancewith some embodiments.

FIGS. 4A-4C illustrate various embodiments of a transparent conductivefilm.

FIG. 5 illustrates a touch-sensitive device having conductive lines madeof a transparent conductive film, in accordance with some embodiments.

FIGS. 6A-6C illustrate examples of conductive line shapes, in accordancewith some embodiments.

FIGS. 7A-7B are a flowchart illustrating a method of fabricating aphotoactive composite conductive film, in accordance with someembodiments.

FIG. 8 is a flowchart illustrating a method of fabricating a compositeconductive film with enhanced hardness, in accordance with someembodiments.

Like reference numerals refer to corresponding parts throughout thedrawings.

DESCRIPTION OF EMBODIMENTS

The embodiments described herein provide composite conductive films thatmay be implemented in connection with many different types of devices,in accordance with various embodiments. These composite conductive filmsinclude a layer of organic material (e.g., a polymer) and an inorganicconductive material (e.g., metallic nanowires). The polymer holds thewires in place mechanically and also allows the junctions between wiresto be embedded within, resulting in a smooth surface (which isbeneficial for building thin-film devices and other applications wheresurface-roughness is an issue). Moreover, the polymer is chosen based onadditional useful properties and/or functionality, resulting incomposite conductive films that are imparted with the same propertiesand/or functionalities as the polymer.

While other properties will be apparent to one of ordinary skill in theart, the present disclosure focuses on three types of compositeconductive films that benefit from properties obtained from the polymer:(1) photo-active transparent conductive films; (2) composite conductivefilms with enhanced hardness/durability; and (3) composite conductivefilms with enhanced temperature stability. These composite conductivefilms may be used in a variety of applications. For example, thin filmsolar cells, displays such as liquid crystal displays (LCDs), organiclight-emitting displays, electrophoretic displays, touch screens (e.g.,capacitive or resistive), anti-fog devices such as in automotive andaeronautic windshields, flexible flat-panel lighting (e.g., usingevaporated and/or printed white organic light-emitting diodes), orphoto-detectors can be formed (and optionally patterned) using materialsand/or approaches as described herein.

Regarding photoactive transparent conductive films, conventionalphotoresists are poor conductors of electricity. One way to improve theelectrical conductivity (hereinafter “conductivity”) of a conventionalphotoresist is to disperse conductive particles throughout thephotoresist. This can be done, for example, by dissolving the conductiveparticles in a solution along with the photoresist prior to applicationof the photoresist (e.g., onto a wafer or a chip). Above a so-called“percolation threshold” (e.g., a percolation threshold density of theconductive particles), an electrically coupled network is formed by theconductive particles through which electrical currents can move,resulting in a low sheet resistance. An exact cutoff sheet resistancebetween electrically conducting and electrically insulating films issomewhat subjective and application specific but, generally speaking, afilm with a sheet resistance less than 100 Ω□⁻¹ (“ohms per square”) maybe considered conducting.

The problem with this approach (essentially loading a photoresist withso much conductive material that it becomes electrically conducting) isthat appropriate conductive materials tend to absorb and/or scatteroptical light, making such photoresist opaque and thus unsuitable forapplications that require transparent conductive films. This is aprimary reason that, despite the expensive nature of tin-doped indiumoxide (ITO) and the difficulty in patterning ITO, ITO remains anindustry standard for conductive transparent films. To be sure, the factthat ITO remains prevalent speaks to the elusive nature of suitablealternatives.

The embodiments described herein, however, provide photoactivetransparent conductive films that are inexpensive and easy to patternusing abbreviated lithographic processes. The photoactive transparentconductive film is a composite conductive film that includes at leasttwo parts: (1) a layer of photoresist material (e.g., forming a matrixmaterial) and (2) an inorganic mesh made of a plurality of particles ofan inorganic material. The plurality of particles of the inorganic meshis embedded, for example, within the very surface of the layer ofphotoresist material to form a conductive film that is photoactive byvirtue of the matrix material being a photoactive photoresist. Byrestricting the inorganic mesh to a surface region of the layer ofphotoresist material, absorption of light is greatly reduced. This isdue to the fact that absorption by the conductive material occurs over alesser extent of the film than if the conductive material was dispersedthroughout the bulk of the film, as well as the fact that thepercolation threshold for a substantially two-dimensional (2D) mesh ofconductive material is less than the percolation threshold for a bulk,or three-dimensional (3D), mesh of conductive material. The lattereffect reduces the density of conductive material needed in the surfaceregion.

Since they are made using a photoresist matrix, these films can bepatterned quite easily. Nearly any arbitrary shape down to sub-micronlength-scales can be realized simply by exposing the film to light of anappropriate wavelength and developing the film in an appropriatedeveloper. The resulting structure can be permanently or functionallyincorporated as electrical components (e.g., wires or conductive lines)into devices such as displays, solar cells, and others. As an addedbenefit, photoresists tend not to be brittle, making the films disclosedherein suitable for flexible electronics. For example, in someembodiments, the composite conductive films described herein arebendable to a bending radius of at least 0.5 millimeters (mm).

Regarding composite conductive films with enhanced hardness anddurability, there are numerous hard plastic materials (e.g., polymers)that can be used to increase the hardness and durability of thecomposite conductive films described herein. These plastic materialsinclude the family of poly-acrylates and acrylated polyurethanes as wellas epoxies based on alkyl amines. In accordance with some embodiments, arespective one of these materials, or a combination, is initiallydeposited onto a substrate as monomers or oligomers and is subsequentlycross-linked by exposure to heat, exposure to ultraviolet radiation,and/or exposure to a chemical catalyst. More specifically, the compositefilm is made conductive by forming a nanowire network or mesh on asacrificial superstrate and then laminating that superstrate onto thefilm of polymer. The polymer is then cross-linked by, for example,exposing it to ultraviolet (UV) radiation. The sacrificial superstrateis removed and the nanowires remain embedded into the surface of thefilm. The hardness of the cross-linked polymer translates into acomposite conductive film that is correspondingly hard.

Hardness, in some circumstances, is quantified using a standardizedpencil hardness test. This test is performed by pushing the tip of apencil of known hardness across the surface of the film with a knownforce and at a fixed angle (e.g., 45 degrees) in the direction of themovement of the pencil. The standards for the standardized pencil testare maintained by the American Society for Testing and Materials (ASTM).When particular pencil test hardness values are given in the presentdisclosure, such values should be interpreted with respect to thepertinent ASTM standard as of the filling date of the presentdisclosure. ASTM Standard D3363, 2011e2, “Standard Test Method for FilmHardness by Pencil Test,” West Conshohocken, Pa., 2011, DOI: 10.1520/D3363-05R11E02, www.astm.org (this standard is herein incorporatedby reference in its entirety).

Regarding composite conductive films with enhanced temperaturestability, the use of polymers such as silicones, polyamides, andpolyimides is utilized in some embodiments to create a compositeconductive film that is heat-stabilized (i.e., the entire compositeconductive film is heat stabilized, not just the polymer itself). Thisis of particular importance because many of the composite conductivefilms described herein make use of a mesh of inorganic nanowires thatprovides the conductivity. But because nanoparticles (e.g., nanowires)exhibit enhanced diffusion of surface atoms, they are less structurallystable than the corresponding bulk material (e.g., bulk metal). Forexample, while bulk silver has a melting point of 961° C., silvernanowires begin to lose structural stability at significantly lowertemperatures, with smaller wires less stable at a given temperature thanlarger ones. Nanowires with an approximate diameter of 100 nanometers(nm) exhibit instability at about 200° C., while nanowires with anapproximate diameter of 25 nm are unstable at 120° C. Above thesetemperatures, the nanowires “bead up” and no longer provide conductivepathways. By embedding the nanowire mesh in a cross-linked polymer, asdescribed below, a nanowire mesh that would ordinarily (e.g., inisolation) be unstable at about 200° C. are stable to at least 350° C.(e.g., as indicated by their good conductive properties at 350° C.). Oneof ordinary skill in the art will appreciate that the exact stabilitytemperature depends on the nature of the nanowires, including theirsize, shape, and composition.

As used herein, the term conducting should be construed to meanelectrically conducting unless clearly stated otherwise.

Reference will now be made in detail to various implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous specific details are set forthin order to provide a thorough understanding of the present disclosureand the described implementations herein. However, implementationsdescribed herein may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andmechanical apparatus have not been described in detail so as not tounnecessarily obscure aspects of the implementations.

FIG. 1A illustrates a perspective view of a device 100-a, in accordancewith some embodiments. Device 100-a includes a substrate 102 thatprovides underlying support for the device 100-a. In some embodiments,the substrate 102 comprises glass or polyethylene terephthalate (PET).Device 100-a further includes distinct instances of a transparentconductive film 104 (e.g., transparent conductive films 104-a and 104-b)that are patterned upon the substrate 102. In some embodiments, thedistinct instances of the transparent conductive film 104 form wires,conductive lines, or other electrical components of the device 100-athat are permanently and/or functionally incorporated into the device100-a (e.g., in its end- or product-ready-state). For example, in someembodiments, the distinct instances of the transparent conductive film104 form conductive lines for a touch screen device or electricalcontacts for a solar cell.

The transparent conductive film 104 comprises a layer of photoresistmaterial 106 (distinct instances of which are 106-a and 106-b). The termphotoresist, as used herein, may be construed to mean any material thatundergoes a structural or chemical change when exposed to apredetermined wavelength of light (or light within a predetermined rangeof wavelengths, such as optical light, ultraviolet light, etc.). Thesechanges can facilitate the removal of undesired portions of thephotoresist material by either (1) making exposed portions insoluble toremoving agents (said removing agents are sometimes called“developers”), in which case the photoresist is considered a negativephotoresist, or (2) making an insoluble portion of the photoresistsoluble when exposed to light, in which case the photoresist isconsidered a positive photoresist. As described in greater detail later,the layer of photoresist material 106 can comprise either a positivephotoresist or a negative photoresist. Selectively exposing thephotoresist material to light thus enables one to fashion thetransparent conductive film 104 according to a complex pattern (e.g.,create distinct instances of the transparent conductive film 104) withhigh resolution (e.g., micron resolution or sub-micron resolution). Insome embodiments, the photoresist material comprises an organicmaterial. In some embodiments the photoresist is a negative photoresist.For example, in some embodiments, the layer of photoresist material 106comprises poly(methyl methacrylate) (PMMA, sometimes called “acrylicglass”). Poly(methyl methacrylate) is photosensitive to deep ultraviolet(UV) light (e.g., light having a wavelength in a range of 200-270nanometers (nm)). Exposure to deep UV light creates chain scissions thatde-crosslink chains of PMMA, making the photoresist soluble in certaindevelopers (thus, PMMA is a positive photoresist). In another example,in some embodiments the layer of photoresist material 106 comprises aphotoresist provided by Kayaku Microchem, such as SU-8, which is anegative photoresist that is photosensitive to near ultraviolet light(e.g., light having a wavelength in a range of 300-400 nm). Otherexamples of negative resists that can be used include, but are notlimited to, azidelisoprene negative resists, polymethylisopropyl ketone(PMIPK), poly-butene-1-sulfone (PBS), poly-(trifluoroethylchloroacrylate) TFECA, copolymer-(V-cyano ethyl acrylate-V-amido ethylacrylate) (COP), poly-(2-methyl pentene-1-sulfone) (PMPS) and the like.It should be understood, however, that the exact nature or chemicalcomposition of the photoresist is not intended to limit the claims thatfollow. In fact, since photoresist are oftentimes proprietary in nature,the exact chemical composition or nature of the photoresist may not beknown when practicing the present disclosure.

In some embodiments the photoresist is a positive photoresist. Thepositive resist is relatively insoluble. After exposure to the properlight energy, the resist converts to a more soluble state. This reactionis called photosobulization. One positive photoresist in accordance withthe present disclosure is the phenol-formaldehyde polymer, also calledphenol-formaldehyde novolak resin. See, for example, DeForest,Photoresist: Materials and Processes, McGraw-Hill, New York, 1975, whichis hereby incorporated by reference herein in its entirety. In someembodiments, the resist layer is LOR OSA, LOR 5 0.7A, LOR1A, LOR 3A, orLOR 5A (MICROCHEM, Newton, Mass.). LOR lift-off resists usepolydimethylglutarimide.

FIG. 1B illustrates a perspective view of a device 100-b, in accordancewith some embodiments. Device 100-b is more general than device 100-a.Device 100-b includes a substrate 102 (analogous to substrate 102 inFIG. 1A) that provides underlying support for the device 100-b. In someembodiments, the substrate 102 comprises glass or polyethyleneterephthalate (PET). Device 100-b further includes a compositeconductive film 140 that is disposed upon the substrate 102. In variousembodiments, composite conductive film 140 has enhancedhardness/durability properties, thermal stability, and/or chemicalstability that are imparted by the particular choice and/or treatment ofa selected polymer 146. In some such cases, polymer 146 is across-linkable polymer (e.g., for applications in which an enhancedsurface hardness is desirable). Specifically, in some embodiments,composite conductive film 140 has enhanced hardness properties (e.g.,the composite conductive film has a pencil test hardness in a range of2H to 9H) because polymer 146 includes polyacrylates, acrylatedpolyurethanes and/or alkyl amines. In some circumstances, the hardnessis somewhat dependent on the nature (e.g., composition) of the substrate102.

Alternatively, or in addition, in some embodiments composite conductivefilm 140 has enhanced thermal stability. This means that the particles110 are, in isolated form, characterized by a first conductivitystability temperature (e.g., a temperature at which they melt or “beadup”) and the composite conductive film (i.e., including the particles110) have a second conductivity stability temperature that is greaterthan the first conductivity stability temperature. For example, in somecircumstances the particles 110 are nanowires with a melting temperatureof 200° C. but, when imbedded in a cross-linked polymer 110, are stableto at least 350° C.

In some embodiments, the particles 110 (e.g., comprising inorganicnanowires) are embedded into the surface 114 such that they make aconductive surface that is accessible to an agent that touches thesurface without needing to penetrate through or into the surface 114. Insome embodiments, the particles 110 are embedded throughout at least aregion of the layer of cross-linked polymer. The region is continuousfrom and/or proximal to the surface 114.

In some embodiments, polymer 146 is a birefringent polymer, in whichcase polymer 146 can be used to make a composite conductive film 140that also, for example, functions as a wave plate for use in LCDscreens, Pockels cells, or other photon-polarization relatedapplications. Birefringent polymers include polyester, polyethylene,nylon, and most polymers when deposited with built-in mechanical stress.

In some embodiments, polymer 146 is chosen for its index of refraction,thus modifying the index of refraction of the composite conductive film140. The index of polymer 146 is chosen to match a substrate 102 or totailor the properties of the composite conductive film 140 to enhanceinternal reflection or light out-coupling at certain wavelengths.

In some embodiments, a thermally insulating material (such as anaerogel) is chosen to be polymer 146. In some embodiments, compositeconductive film 140 is used to make conductive materials that do notsubstantially transmit heat (or, stated another way, resist heattransfer). In some embodiments, composite conductive film 140 is used,e.g., in transparent displays used on refrigerator cases.

In some embodiments, a porous polymer is chosen to be polymer 146. Insome embodiments, conductive composite film 140 is conductive yet allowsgas to flow through it, e.g., for use in active contact lenses.Conductive composite films 140 made using porous polymers as polymer 146can be designed to have specific surface chemistries to suitapplications in chemical sensors.

In some embodiments, polymer 146 is chosen so as to impart chemicalstability to the composite conductive film 140. For example, somepolymers act to keep water from the nanowire surface by taking advantageof their hydrophobicity. Other polymers preferentially react with oxygenor sulfides so that those compounds are not available to react with thenanowires. As a specific example, in some embodiments, polymer 146 ischosen to comprise a polyacrylate (e.g., poly-methylmethacrylate)material so as to prevent oxidation of the composite conductive film140, where the composite conductive film 140 includes a mesh of silver(Ag) nanowires. Because oxidation degrades the conductivity of thecomposite conductive film, loss of conductivity can be used as a metricto evaluate oxidative degradation of the silver network over time.

In the remaining description, certain properties are referred to thatare applicable to either device 100-a or device 100-b. Thus, photoresistmaterial 106 and polymer 146 are collectively referred to as “material106/146”; while transparent conductive film 104 and composite conductivefilm 140 are collectively referred to as “composite conductive film104/140.” The descriptions of individual properties below are intendedto be applicable to device 100-a and/or device 100-b, either alone or incombination with any other set of properties described below.

In some embodiments, material 106/146 is an electrically insulatingmaterial (e.g., the layer of material 106/146 has a sheet resistanceabove 5000 Ω□⁻¹).

In some embodiments, the layer of material 106/146 has a controllablethickness (e.g., a thickness that is controlled by process parametersduring fabrication). For example, in some embodiments, the substrate 102is a chip or a wafer and the layer of material 106/146 is applied to thechip or wafer by spin coating the material (e.g., polymer and/orphotoresist) onto the substrate 102 and subsequently baking the material(e.g., to evaporate solvent). In such embodiments, the resultingthickness of the layer of material 106/146 depends on an angular speedof the spin coater (e.g., as measured in revolutions per minute) as wellas the composition and viscosity of the material in dissolved form. Insome embodiments, the thickness of the layer of material 106/146 is in arange of 100-500 nm, 400-1000 nm, 900-1500 nm, or 1-10 microns. In someembodiments, the layer of material 106/146 is deposited onto thesubstrate using another deposition technique such as the Spin-On-Glassmethod (SOG). More information on SOG can be found, for example, inNguyen Nhu Toan, Spin-On Glass Materials and Applications in Advanced ICTechnologies, 1999, which is hereby incorporated herein by reference inits entirety.

In some embodiments, the layer of material 106/146 is deposited onto thesubstrate using ink-jet printing. Ink-jet printing is based on the sameprinciples of commercial ink-jet printing. The ink-jet nozzle isconnected to a reservoir filled with the chemical solution and placedabove a computer-controlled x-y stage. The target object is placed onthe x-y stage and, under computer control, liquid drops (e.g., 50microns in diameter) are expelled through the nozzle onto a well-definedplace on the object. Different nozzles print different spots inparallel. In one embodiment of the present disclosure, a bubble jet,with drops as small as a few picoliters, is used to form a layer of thematerial 106/146. In another embodiment, a thermal ink jet (HewlettPackard, Palo Alto, Calif.) is used to form a layer of a depositmaterial. In a thermal ink jet, resistors are used to rapidly heat athin layer of liquid ink. A superheated vapor explosion vaporizes a tinyfraction of the ink to form an expanding bubble that ejects a drop ofink from the ink cartridge onto the substrate. In still anotherembodiment of the present disclosure, a piezoelectric ink-jet head isused for ink-jet printing. A piezoelectric ink-jet head includes areservoir with an inlet port and a nozzle at the other end. One wall ofthe reservoir consists of a thin diaphragm with an attachedpiezoelectric crystal. When voltage is applied to the crystal, itcontracts laterally, thus deflecting the diaphragm and ejecting a smalldrop of fluid from the nozzle. The reservoir then refills via capillaryaction through the inlet. One, and only one, drop is ejected for eachvoltage pulse applied to the crystal, thus allowing complete controlover when a drop is ejected. In yet another embodiment of the presentdisclosure, an epoxy delivery system is used to deposit material106/146. An example of an epoxy delivery system is the Ivek Digispense2000 (Ivek Corporation, North Springfield, Vt.). For more information onjet spraying, see, for example, Madou, Fundamentals of Microfabrication,Second Edition, 2002, pp. 164-167, CRC Press, which is herebyincorporated by reference herein in its entirety.

In another embodiment of the present disclosure, the layer of material106/146 is deposited onto the substrate by a screen printing (also knownas silk-screening) process. A paste or ink is pressed onto portions ofthe substrate through openings in the emulsion on a screen. See, forexample, Lambrechts and Sansen, Biosensors: MicroelectrochemicalDevices, The Institute of Physics Publishing, Philadelphia, 1992, whichis hereby incorporated by reference in its entirety. The paste consistsof a mixture of the material of interest, an organic binder, and asolvent. The organic binder determines the flow properties of the paste.The bonding agent provides adhesion of particles to one another and tothe substrate. The active particles make the ink a conductor, aresistor, or an insulator. The lithographic pattern in the screenemulsion is transferred onto portions of the underlying structure byforcing the paste through the mask openings with a squeegee. In a firststep, paste is put down on the screen. Then the squeegee lowers andpushes the screen onto the substrate, forcing the paste through openingsin the screen during its horizontal motion. During the last step, thescreen snaps back, the thick film paste that adheres between thescreening frame and the substrate shears, and the printed pattern isformed on the substrate. The resolution of the process depends on theopenings in the screen and the nature of the paste. With a 325-meshscreen (i.e., 325 wires per inch or 40 μm holes) and a typical paste, alateral resolution of 100 μm can be obtained.

For difficult-to-print pastes, a shadow mask, such as a thin metal foilwith openings, complements the process. However, the resolution of thismethod is inferior (>500 μm). After printing, the wet films are allowedto settle for a period of time (e.g., fifteen minutes) to flatten thesurface while drying. This removes the solvents from the paste.Subsequent firing burns off the organic binder, metallic particles arereduced or oxidized, and glass particles are sintered. Typicaltemperatures range from 500° C. to 1000° C. After firing, the thicknessof the resulting layer ranges from 10 μm to 50 μm. One silk-screeningsetup is the DEK 4265 (Universal Instrument Corporation, Binghamton,N.Y.). Commercially available inks (pastes) that can be used in thescreen printing include conductive (e.g., Au, Pt, Ag/Pd, etc.),resistive (e.g., RuO₂, IrO₂), overglaze, and dielectric (e.g., Al₂O₃,ZrO₂). The conductive pastes are based on metal particles, such as Ag,Pd, Au, or Pt, or a mixture of these combined with glass. Resistivepastes are based on RuO₂ or Bi₂Ru₂O₇ mixed with glass (e.g., 65% PBO,25% SiO₂, 10% Bi₂O₃).

The resistivity is determined by the mixing ratio. Overglaze anddielectric pastes are based on glass mixtures. Different meltingtemperatures can be achieved by adjusting the paste composition. See,for example, Madou, Fundamentals of Microfabrication, Second Edition,CRC Press, Boca Raton, Fla., 2002, pp. 154-156, which is herebyincorporated by reference herein in its entirety.

In some embodiments, the composite conductive film 104/140 is atransparent conductive film characterized by a low index of refraction(e.g., in a range of 1.5-2), obviating the need for anti-reflectivecoatings in certain applications (i.e., as compared to ITO when used inthe same applications).

The composite conductive film 104/140 includes an inorganic mesh 108embedded within the layer of material 106/146. The inorganic mesh 108includes a plurality of particles 110 of an inorganic material. As shownin FIG. 1A, particle 110-a and particle 110-b are respective particlesof the plurality of particles. For visual clarity, the remainingparticles in FIGS. 1A-1B are not labeled. In some embodiments, therespective particles of the plurality of particles 110 comprise aconductive material. For example, in various embodiments, the pluralityof particles 110 comprises inorganic nanowires (as shown in FIGS.1A-1B), nanotubes, nanospheres, graphene, or a heterogeneous mixturethereof. In some embodiments, the nanowires are silver (Ag) nanowires.More generally, in some embodiments, the nanowires are metallicnanowires. In some embodiments, the nanotubes comprise carbon nanotubes(CNT). In some embodiments, the inorganic mesh 108 comprises aheterogeneous mixture of particles (e.g., a mixture of carbon nanotubesand graphene particles). In some embodiments, each nanowire or nanotubeis characterized by a major axis and one or more minor axes. The majoraxes are aligned substantially parallel to a surface 114 of the layer ofmaterial 106/146. A typical width of a nanowire (i.e., along arespective minor axis of the nanowire) will be on the order of tens ofnanometers, while the length (i.e., along the major axis of thenanowire) is often on the order of hundreds of nanometers or microns. Insome embodiments, the nanowires have an average aspect ratio between10-1000.

In some embodiments, the respective particles of the plurality ofparticles 110 are fused (e.g., electrically coupled) to form asubstantially continuous network of the conductive material over acontinuous portion of the composite conductive film (e.g., therespective particles of the plurality of particles 110 are fused to forma substantially continuous network over the conductive material overeach instance of the composite conductive film 104/146). In other words,the respective particles of the plurality of particles 110 are fused inthe sense that they form a continuous network of the conductivematerial. For example, the respective particles of the plurality ofparticles 110 may still be considered fused if the network includesdead-ends or if there are some particles or groups of particles that arenot continuous with the network (e.g., islands of particles).

In some embodiments, the substantially continuous network of theconductive material is confined to a surface region 112 of the layer ofmaterial 106/146 (e.g., the surface region 112 has a thickness that isthin compared to the thickness of the layer of material 106/146),thereby forming a substantially two-dimensional continuous network ofthe conductive material proximal to the surface 114 of the layer ofmaterial 106/146. For example, in some circumstances, the thickness ofthe surface region 112 is 10, 20, 50, or 100 times thinner than theoverall thickness the layer of material 106/146 (e.g., the thickness ofthe surface region 112 is 25 nm while the overall thickness of the layerof material 106/146 is 1 micron, giving a ratio of the thickness of thesurface region 112 to the overall thickness of the layer of material106/146 of 40).

More generally, the plurality of particles 110 (e.g., inorganicnanowires) is embedded throughout at least a region of the layer ofmaterial 106/146. The region should be construed to be a region in two-or three-dimensions unless otherwise specified. That is to say, in someembodiments, the region has a “footprint” that encompasses less than theentire surface area of the layer of cross-linked polymer and has a depththat is less than the entire depth of the layer of cross-linked polymer.In some embodiments, the region in which the inorganic mesh is embeddedis restricted to a sub-layer of the layer of cross-linked polymer, wherethe sub-layer is continuous with the surface.

By confining the plurality of particles 110 to the surface region 112(e.g., proximal to surface 114), some embodiments take advantage of thefact that the composite conductive film 104/140 can confer conductivityto surface region 112 using far fewer particles than would be otherwiserequired to make the material 106/146 as a whole conductive throughdispersal of the particles throughout the bulk of the photoresistmaterial. The reason for this is two-fold: (1) even if particles had thesame density in both cases, fewer particles would be needed to achievethat density in a smaller region (e.g., the surface region 112 ascompared to the bulk) and (2) the percolation threshold is lower for atwo-dimensional (2D) network of particles than it is for a bulk, orthree-dimensional (3D), network of particles. Thus, in some embodiments,the plurality of particles 110 is characterized by a density within thesurface region 112 that is above the two-dimensional percolationthreshold (corresponding to a 2D network of particles) and less than thebulk percolation threshold of the conductive material suspendedthroughout the layer of material 106/146. This is particularlyadvantageous in certain circumstances (e.g., when a high-degree oftransparency is needed) because, in some embodiments, the respectiveparticles of the plurality of particles 110 comprise metallic particlesthat tend to absorb and/or scatter optical light.

In some embodiments, the layer of material 106/146 is substantiallytransparent to optical light (e.g., light having a wavelength within arange of 400-700 nm). As used herein, the term “transparent” may beunderstood to refer to characteristics relating to the passage of atleast some light as measured with respect to light from a light sourcehaving a predetermined spectral density. In some embodiments, the lightsource is the sun. Alternatively, the light source is a light sourcethat emits a narrow band of light (e.g., a laser or light that haspassed through a band pass filter). Alternatively, the light source is acommon household light source, such as an incandescent light bulb or acompact fluorescent lamp (CFL). Regardless of the type of light sourcethat is used to define transparency, different transparent materials (inaccordance with different embodiments) exhibit different levels oftransparency. For example, in some embodiments, the composite conductivefilm is characterized by a transparency to optical light in one or moreof the following ranges: 60%-100%, 60%-99%, 60%-90%, and greater than50%. In many optoelectronic implementations, the transparent conductivefilms pass between about 60% and 90% of incident light, as dictated bythe application.

FIGS. 2A-2E illustrate a method 200 of fabricating (e.g., manufacturing)a composite conductive film (e.g., composite conductive film 104/140,FIGS. 1A-1B), in accordance with some embodiments. In variousembodiments, the composite conductive film is a photoactive transparentconductive film and/or a film with enhanced surface properties (e.g.,enhanced hardness and/or temperature stability). For ease ofexplanation, the method 200 is described with reference to a nanowiremesh (e.g., inorganic mesh 108, FIG. 1A) comprising a plurality ofnanowires made of a conductive material. It should be understood,however, that the method 200 can be implemented using a variety ofconductive materials, including but not limited to conductive nanowires,nanotubes, nanospheres, graphene, and/or a heterogeneous mixturethereof.

FIG. 2A illustrates an operation in which a nanowire suspension 202(e.g., comprising a solution of nanowires suspended in a solvent) isapplied to (e.g., drop-cast onto) a surface 204 of a glass transferblock 206. The nanowire suspension 208 is applied, for example, in someembodiments, by mixing nanowires in the solution and coating ordepositing the solution on the surface 204 of the glass transfer block206. In alternate embodiments, the transfer block may comprise amaterial other than glass.

In FIG. 2B, the nanowire suspension 202 is dispersed on the surface 204to form a nanowire mesh 208. In some embodiments, the nanowiresuspension 202 is dispersed by spin coating the nanowire suspension 202onto the glass transfer block 206. In some embodiments, the nanowiresuspension 202 is dispersed by any of the deposition techniquesdisclosed or referenced herein. The nanowire suspension 202 issubsequently dried to evaporate the solvent leaving behind the nanowiremesh 208. In some embodiments, to prevent nanowire aggregation, thenanowire suspension 202 is dried slowly (e.g., at room temperature)while the glass transfer block 206 is shaken (e.g., moved laterallyand/or angularly at a frequency between 1-60 Hz). The nanowire mesh 208is optionally annealed (e.g., by raising the temperature of the glasstransfer block 206 to about 180° C. for a period of about 1 hour) tofurther fuse (e.g., electrically couple) the nanowires and form asubstantially continuous network of the conductive material, therebyreducing sheet resistance of the final transparent conductive film.

FIG. 2C illustrates an operation in which the surface 204 of the glasstransfer block 206 is pressed onto a surface 114 of a layer of organicmaterial 216, which is disposed upon an underlying substrate 102 (e.g.,glass or PET). In some embodiments, the organic material is aphotoresist. In some embodiments, the organic material 216 iscross-linkable but in a substantially noncross-linked state. In someembodiments, the surface 204 of the glass transfer block 206 is pressedonto the surface 114 of the layer of organic material 216 with apredetermined (e.g., calibrated) pressure (e.g., a pressure of 2.4×10⁴psi). The method 200 results in a device 210 that includes a nanowiremesh 108 embedded in the layer of organic material 216, thus forming thecomposite conductive film 104/140.

In some embodiments, the organic layer 216 is a polymer. In someembodiments, the polymer is an electrically insulating material (e.g.,the layer of organic material 216 has a sheet resistance above 5000Ω□⁻¹). Upon embedding the nanowire mesh 108 in the layer of organicmaterial 216, the composite conductive film 104/140 becomes conducting(e.g., with a sheet resistance less than 100 Ω□⁻¹ or 200 Ω□⁻¹).

By applying pressure to the nanowire mesh 108 in this manner, amechanical force is applied to the nanowires. The mechanical force fusesthe nanowires together (e.g., where they are in contact with oneanother) and fills in the spaces between nanowires with photoresist.Thus, the application of pressure serves to improve electricalconductivity and reduce surface roughness for the resulting device 210.This, in turn, affects both optical and electrical properties of thedevice 210. For example, the haze of a transparent conductive film(e.g., transparent conductive film 104, FIG. 1A) is proportional to thetransparent conductive film's surface roughness because the stacking andprojection of the nanowires increases the transparent conductive film'stendency to absorb and/or scatter light. In addition, when the nanowiresbecome fused, the electrical resistance of the transparent conductivefilm 104 substantially decreases.

As a more detailed example, in some implementations, the nanowires inthe nanowire suspension 202 have varying diameters (e.g., rangingbetween 50-100 nm). Accordingly, diameters of nanowires in the nanowiremesh 208 (e.g., prior to embedding the nanowire mesh 208 in the layer oforganic material 216) may vary from one another, with differencesupwards of 50 nanometers. In some embodiments, after the surface 204 ofthe glass transfer block 206 is pressed onto the layer of organicmaterial 216, a composite conductive film 104/140 having a surfaceroughness that is less than 40 nanometers root-mean-square (RMS) isachieved. In other implementations, the surface roughness is less thanabout 20 nanometers RMS. More generally, in some embodiments, thecomposite conductive film 104/140 is characterized by a first surfaceroughness that is less than a second surface roughness of a stand-alonefilm (e.g., nanowire mesh 208) consisting of the same thickness of theinorganic mesh. For example, the first surface roughness is about fiftypercent less than the second surface roughness.

In some embodiments, one or more parameters (e.g., the applied pressureand/or the composition, length, and/or density of the nanowires) can bevaried to deliberately alter (e.g., tune) the transparency and haze ofthe device 210 for specific applications (e.g., by varying the thicknessof surface region 112, FIG. 1A or the surface roughness). Alternatively,or in addition to, in some embodiments, the thickness of the surfaceregion 112 (FIG. 1A and/or FIG. 1B) is selected in accordance with atarget conductivity value.

FIG. 2D illustrates an optional operation in which the device 210 isexposed to heat (e.g., cured or annealed using an oven or hot plate 218)in order to cross-link the organic material 216 (e.g., for applicationsin which an enhanced surface hardness and/or temperature stability isdesirable). The exact temperature or range of temperatures at which toheat device 210 depends on the nature of the organic material 216. Insome embodiments, the organic material 216 is a polyamide or polyimidethat undergoes cross-linking at a temperature that is below its glasstransition temperature. For example, some polyimides undergo substantialcross-linking at 350° C.-400° C. The length of exposure to elevatedtemperature is less critical, but between 1-30 minutes providereasonable values. In some embodiments, after curing, the compositeconductive film is baked for an additional amount of time to evaporateresidual solvents. The hardness of the cross-linked organic material 216translates into a composite conductive film 104/140 that iscorrespondingly hard.

FIG. 2E illustrates an optional alternate operation for cross-linkingpolymers within organic material 216. Namely, FIG. 2E illustrates anoperation in which the device 210 is exposed to ultraviolet (UV)radiation (e.g., using a UV source 220) in order to cross-link theorganic material 216 (e.g., for applications in which an enhancedsurface hardness and/or temperature stability is desirable). In someembodiments, a dose of between 6-12 Mrad of UV radiation is impingedupon the surface 112 of the composite conductive film 104/140 to achievethe cross-linking. In some embodiments, the cross-linking is improved bymixing the polymer with a suitable unsaturated radiation cross-linkingagent (sometimes called a “pro-rad”).

FIGS. 3A-3C illustrate a method 300 of patterning a transparentconductive film 104 using an abbreviated lithographic process, inaccordance with some embodiments.

In FIG. 3A, a photomask 302 is positioned with respect to the conductivesurface of a photoactive transparent conductive film 104 (e.g., thephotomask 302 is either in contact with the photoactive transparentconductive film 104 or separated from the photoactive transparentconductive film 104 by a predetermined distance). In some embodiments,the photomask is a stepper reticle and, when in use, one or more opticalelements (not shown) are disposed between photomask 302 and thetransparent conductive film 104. In other embodiments, the photomask 302is a contact printing photomask and, when in use, photomask 302 is incontact with or very close to (e.g., 1 micron, or 2 microns away from)the transparent conductive film 104. Transparent conductive film 104 isdescribed herein as a “photoactive” transparent conductive film because,prior to outset of method 300, the photoresist included in thetransparent conductive film 104 has not been exposed to light to whichthe photoresist is sensitive. The photomask 302 has one or more opaqueregions 304 that block light and one or more transparent region 306which allow light to pass onto the surface 114 of the photoactivetransparent conductive film 104.

The photomask 302 is illuminated with light 308 having a predeterminedwavelength or range of wavelengths. The photoactive transparentconductive film 104 is sensitive to light of the predeterminedwavelength or range of wavelengths. The photomask 302 is illuminated inthis manner with a predetermined intensity for a predetermined amount oftime in accordance with process requirements and recipes. In someembodiments, the predetermined wavelength is in a range of ultraviolet(UV) wavelengths (e.g., 200-270 nm for deep-UV, or 300-400 nm fornear-UV). In some embodiments, the predetermined wavelength is distinctfrom (e.g., outside a range of) wavelengths for which the transparentconductive film 104 is intended to be transparent.

The transparent regions 306 result in the exposure of selected regions310 (e.g., regions 310-a, 310-b, 310-c, and 310-d) of the photoactivetransparent conductive film 104 to light having the predeterminedwavelength. In some embodiments, the photoresist material is a positivephotoresist. The positive resist is relatively insoluble. After exposureto the proper light energy, the resist converts to a more soluble state.This reaction is called photosobulization. In some embodiments, thephotoresist material is a positive photoresist that is degraded byexposure of light having the predetermined wavelength. For example, thephotoresist, prior to exposure to light, is characterized bycross-linking of polymers. The cross-linking of polymers is broken inthe first region (e.g., scissions are formed and the polymers arede-crosslinked) to form a noncross-linked state by exposure to lighthaving the predetermined wavelength.

FIG. 3B illustrates the result of the method 300 when a positivephotoresist is used. When a positive photoresist is used, the selectedregions 310 are removed upon application of a developer to form, forexample, trenches 312 (e.g., trenches 312-a, 312-b, 312-c, and 312-d),while the unexposed regions are not removed.

In some embodiments, the photoresist is a negative photoresist in whichpolymers in the resist form a cross-linked material that is etchresistant upon exposure to light. In some embodiments, the photoresistis a negative photoresist comprising a polymer and exposing the selectedregions 310 of the photoactive transparent conductive film 104 to lighthaving the predetermined wavelength results in crosslinking of thepolymer. When a negative photoresist is used, the selected regions 310remain upon application of a developer, while unexposed regions areremoved.

FIG. 3C illustrates the result of the method 300 when a negativephotoresist is used. When a negative photoresist is used, the selectedregions 310 remain (i.e., are not removed) upon application of adeveloper to form, for example, pillars 314 (e.g., pillars 314-a, 314-b,314-c, and 314-d), while the unexposed regions are removed.

In some circumstances (e.g., for some photoresists), the cross-linkingof the polymer is driven by thermal processes activated by the UVradiation (e.g., the UV radiation locally increases the temperature ofthe photoresist, which drives the cross-linking process). In somecircumstances, the transparent conductive film 104 does not include aphotoresist, but is nonetheless a layer of a material that undergoescross-linking when exposed to elevated temperature (e.g., thermalcross-linking). Such materials can be beneficial when mechanicalrobustness is desired (e.g., without the need for patterning). In someembodiments, the transparent conductive film 104 includes anelectron-beam (e-beam) sensitive resist in lieu of a photoresist(however, PMMA is considered both an e-beam resist and a photoresistsensitive to UV light). In some embodiments, the transparent conductivefilm 104 is deposited using monomers or oligomers in solution, and acatalyst is added to the solution to facilitate cross-linking (e.g.,prior to depositing the solution onto the substrate, drying the solutionto form the layer, and embedding the wires into the surface of thelayer).

Although removal of the unwanted regions of the photoresist (e.g., theunexposed regions or, alternatively, the exposed regions, depending onthe nature of the photoresist) has been described with reference to useof a developer, some embodiments utilize a mechanical approach toremoving such regions. Such mechanical approaches can include an appliedforce or blasting with a fluid such as air or water.

FIGS. 4A-4C illustrate alternate embodiments of a transparent conductivefilm. In some embodiments, the layers of photoresist material 106 shownin FIGS. 4A-4C are disposed upon a substrate (e.g., substrate 102, FIG.1A), although the substrate is not shown here for visual clarity. Thetransparent conductive films illustrated in FIG. 4A-4C are intended tobe only exemplary and are not intended to limit the claims that follow.

FIG. 4A illustrates a transparent conductive film 402. The transparentconductive film 402 is a composite conductive film that includes a layerof photoresist material 106 and an inorganic mesh comprising a pluralityof nanospheres 404 (e.g., metallic nanospheres). The inorganic mesh isconfined to a surface region 406 of the transparent conductive film 402,which is advantageous in that fewer nanospheres 404 are required toachieve a suitable sheet resistance, resulting in improved transparencycharacteristics.

FIG. 4B illustrates a transparent conductive film 408. In an analogousmanner to FIG. 4A, the transparent conductive film 408 is a compositeconductive film that includes a layer of photoresist material 106 and aninorganic mesh comprising a plurality of nanospheres 404 (e.g., metallicnanospheres). Unlike in FIG. 4A, however, the inorganic mesh is notconfined to a surface region of the transparent conductive film 408 andis instead suspended throughout the bulk of the layer of photoresistmaterial 106. The transparent conductive film 408 can be used when ahigh degree of transparency is not needed (or a hazy film is desired) orwhen a high degree of conductivity is not needed.

FIG. 4C illustrates a transparent conductive film 410. FIG. 4C isanalogous to FIG. 4B but for the fact that transparent conductive film410 includes an inorganic mesh comprising a plurality of nanowires 110suspended throughout the bulk of the layer of photoresist material 106.

FIG. 5 illustrates a touch-sensitive device 500 having conductive linesof a transparent conductive film, in accordance with some embodiments.FIG. 5 illustrates several layers, panels, and/or components of thetouch-sensitive device 500. For visual clarity, these layers areillustrated as separated in the vertical direction. More typically,these layers will be connected to one another or, alternatively, coupledwith one another (e.g., with spacer layers in between).

The touch-sensitive device 500 includes a driving panel 502 which has aplurality of conductive driving lines 504 patterned upon a surface ofthe driving panel 502 (e.g., a top surface). Each of the plurality ofconducting driving lines 504 can have any of the properties or featuresof the distinct instances of the transparent conductive film 104described with reference to any of the other figures (e.g., FIG. 1A). Insome embodiments, the device 100-a illustrated in FIG. 1A is a partialcross-section of driving panel 502. In particular, each of the pluralityof driving lines 504 comprises a layer of photoresist material and aninorganic mesh that includes a plurality of particles embedded withinthe layer of photoresist material.

The touch-sensitive device 500 further includes a sensing panel 506electrically coupled with the driving panel 502 by an electricalcharacteristic (e.g., capacitance, resistance) having a value. Thesensing panel 506 includes a plurality of sensing lines 508 disposedupon a surface of the sensing panel. In some embodiments, the sensinglines 508 are analogous to the driving lines 504, except that thesensing lines 508 run in a different direction (e.g., substantiallyperpendicularly) to the driving lines 504 such that the two sets oflines form a grid.

The touch-sensitive device 500 further includes electronic circuitry todetect, using the plurality of driving lines 504 and the plurality ofsensing lines 508, a touch input by sensing a change in the value of theelectrical characteristic (e.g., capacitance or resistance) between thedriving panel 502 and the sensing panel 506.

Finally, the touch-sensitive device 500 includes a display 510 (e.g., anLCD display). The display 510 produces light that is shone upwardsthrough the driving panel 502 and sensing panel 506.

In some embodiments, the conductive lines are straight lines.Alternatively, the conductive lines (e.g., driving lines 504 or sensinglines 508) can have an arbitrary shape.

FIGS. 6A-6C illustrate examples of conductive line shapes, in accordancewith some embodiments. FIG. 6A illustrates conductive lines 602comprising a plurality of rectangles 604 separated by straight segments606 of the transparent conductive film. FIG. 6B illustrates conductivelines 608 comprising a plurality of diamonds 610 separated by straightsegments 606 of the transparent conductive film. FIG. 6C illustratesconductive lines 612 comprising a plurality of circles 614 separated bystraight segments 606 of the transparent conductive film.

FIGS. 7A-7B are a flowchart illustrating a method 700 of fabricating acomposite conductive film. The method 700 can be used to fabricate thecomposite conductive film shown, for example, in FIG. 1A, and/or theconductive driving lines/conductive sensing lines shown in FIG. 5 andFIG. 6.

The method 700 includes providing (702), as a matrix, a layer ofphotoresist material. For example, in some embodiments, the layer ofphotoresist material is spin-coated onto a glass substrate (e.g.,substrate 102, FIG. 1A). In some embodiments, the photoresist materialis an electrically insulating material (e.g., with a sheet resistanceabove 5000 Ω□⁻¹).

The method 700 further includes introducing (704) a plurality ofinorganic particles upon a surface (e.g., a top surface) of the layer ofphotoresist material. In some embodiments, the plurality of inorganicparticles comprise one of nanowires (as shown in FIG. 1A), nanotubes,nanospheres, graphene, or a combination (e.g., heterogeneous mixture)thereof. In some embodiments, introducing the plurality of inorganicparticles upon the surface of the layer of photoresist materialcomprises spraying (706) a nanowire suspension onto the surface of thelayer of photoresist material. In some embodiments, introducing theplurality of inorganic particles upon the surface of the layer ofphotoresist material comprises transferring (708) said plurality ofinorganic particles from a stamp (e.g., glass transfer block 206, FIG.2) in a stamp transfer process (e.g., by the process shown in FIGS.2A-2C). In some embodiments, prior to introducing the plurality ofinorganic particles upon the surface of the layer of photoresistmaterial, the method 700 includes annealing (710) the plurality ofinorganic particles on the stamp to form a substantially continuousconductive network of the inorganic particles. For example, the stampcan be raised to an elevated temperature of 180° C. for a predeterminedperiod of time (e.g., 1 hour) to anneal the plurality of particles.

In some embodiments, prior to embedding the inorganic particles into thelayer of photoresist material (see operation 714), the method 700includes heating (712) the layer of photoresist material from a firsttemperature (e.g., room temperature, or 25° C.) to a second temperature(e.g., 90° C.) greater than the first temperature. The layer ofphotoresist material is softer at the second temperature than at thefirst temperature, thereby facilitating the embedding operation 714).

The method 700 includes embedding (714) at least some of the pluralityof inorganic particles into the layer of photoresist material to form aninorganic mesh within the layer of photoresist material, thereby formingthe composite conductive film. In some embodiments, embedding the atleast some of the plurality of inorganic particles into the layer ofphotoresist material to form the inorganic mesh within the layer ofphotoresist material comprises pressing (716) the nanowire suspensioninto the surface of the layer of photoresist material (e.g., by pressingthe stamp into the surface of the layer of photoresist material with apressure of 2.4×10⁴ psi).

In some embodiments, the method 700 includes patterning (718) thecomposite conductive film by exposing (720) at least a first region ofthe composite conductive film to light having a wavelength. Thephotoresist material is cross-linked by exposure to light having thewavelength (e.g., the photoresist material is a negative photoresist).In some embodiments, patterning the composite conductive film furthercomprises removing (722) at least a second region of the compositeconductive film using a chemical developer. The second region of thecomposite conductive film has not been exposed to light having thewavelength and the photoresist material is soluble in the chemicaldeveloper when the photoresist material is in a noncross-linked state.In some embodiments (724), the wavelength is in a range of ultravioletwavelengths (e.g., in a range of 200-270 nm for deep-UV or 300-400 fornear-UV).

In some embodiments, the method further includes patterning (726) thecomposite conductive film by exposing (728) at least a first region ofthe composite conductive film to light having a wavelength. Thephotoresist material is characterized by cross-linking of polymers andsaid cross-linking of polymers is broken to form a noncross-linked stateby exposure to light having the wavelength (e.g., the photoresistmaterial is a positive photoresist). In some embodiments, patterning thecomposite conductive film further comprises removing (730) the firstregion of the composite conductive film using a chemical developer. Thephotoresist material is soluble in the chemical developer when thephotoresist material is in the noncross-linked state. In someembodiments (732), the wavelength is in a range of ultravioletwavelengths (e.g., in a range of 200-270 nm for deep-UV or 300-400 fornear-UV).

FIG. 8 is a flowchart illustrating a method 800 of fabricating acomposite conductive film with enhanced hardness.

The method includes providing (802), as a matrix, a layer ofcross-linkable polymer. The cross-linkable polymer is in a substantiallynoncross-linked state. In some embodiments, providing, as the matrix,the layer of cross-linkable polymer includes depositing (804) the layerof cross-linkable material on a substrate. Thus, in some embodiments,the composite conductive film is disposed on a substrate. In variousembodiments, the substrate is one of a polyester substrate or aborosilicate glass substrate. The cross-linkable polymer deposited onthe substrate is sometimes called a “precursor.” Various methods ofdepositing the layer of cross-linkable material on the substrate (e.g.,spin-coating) are described elsewhere in this document. In someembodiments, the cross-linkable polymer is deposited as a liquid film.In some embodiments, the cross-linkable polymer is a polyamide or apolyimide. In some embodiments, the cross-linkable polymer is one of anacrylated polyurethane or an epoxy-based alkyl-amine.

The method further includes introducing (806) a plurality of inorganicnanowires onto a surface of the layer of cross-linkable polymer. Forexample, the plurality of inorganic nanowires are sometimes formed on asacrificial superstrate (e.g., glass transfer block 206) and thenlaminated onto the liquid film of the precursor. The method furtherincludes embedding (808) at least some of the plurality of inorganicnanowires into the layer of cross-linkable polymer to form an inorganicmesh within the layer of cross-linkable polymer, thereby forming thecomposite conductive film. In some embodiments, the operation ofintroducing the inorganic nanowires and embedding the inorganicnanowires are one and the same. For example, the superstrate is placedin contact with the liquid film thereby embedding the nanowires withinthe liquid film.

In some embodiments, the respective inorganic nanowires of the pluralityof inorganic nanowires of the inorganic mesh comprise a material havinga conductivity between 1×10⁵ S/cm and 1×10⁶ S/cm, or greater. Forexample, in some embodiments, the respective inorganic nanowirescomprise silver (Ag) with a conductivity of approximately 6×10⁵ S/cm.The plurality of nanowires has an average aspect ratio between 10-1000.

The method further includes cross-linking (810) the cross-linkablepolymer within at least a surface portion of the composite conductivefilm. For example, the cross-linkable polymer comprises is a materialthat undergoes cross-linking when exposed to one of ultraviolet (UV)radiation, heat treatment, or a chemical catalyst. Following thecross-linking, the cross-linkable polymer within at least the surfaceportion of the composite conductive film is in a cross-linked state.Cross-linking the cross-linkable polymer within at least the surfaceportion of the composite conductive film results in a surface penciltest hardness of the composite conductive film that is between 2H and9H.

In some embodiments, a method is provided that is analogous to method800 except that, rather than (or in addition to) producing a compositeconductive film with enhanced hardness properties, the operation ofcross-linking the cross-linkable polymer within at least the surfaceportion of the composite conductive film results in at least the surfaceportion of the composite conductive film having a second conductivitystability temperature that is greater than a first conductivitystability temperature, where the nanowires of the inorganic materialare, in isolated form, characterized by the first conductivity stabilitytemperature. In some embodiments, the first conductivity stabilitytemperature of the nanowires is a threshold that, when surpassed, causesa substantial reduction in the conductivity of the isolated nanowiremesh (e.g., causes a phase transition). In some circumstances, thisreduction in conductivity persists even when the temperature issubsequently reduced below the first conductivity stability temperature.In some embodiments, the second conductivity is a threshold that, whensurpassed, causes a substantial reduction the conductivity of theembedded nanowire mesh (e.g., causes a phase transition). In someembodiments, the first conductivity stability temperature is less than abulk conductivity stability temperature (e.g., a bulk meltingtemperature) of the inorganic material (e.g., silver). In someembodiments, the first conductivity stability temperature is atemperature at which the nanowires change shape (e.g., “bead up”).

In some embodiments, the composite conductive film is characterized by asheet resistance having a temperature coefficient (e.g., an averagetemperature coefficient) that is less than 0.002 (ΩK)⁻¹ between thefirst conductivity stability temperature and the second conductivitystability temperature.

In some embodiments, the layer of cross-linked polymer comprises one ofa silicone material, a polyamide material, or a polyimide material.

The foregoing description, for purposes of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated.

It will be understood that, although the terms “first,” “second,” etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first instance of a layer ofmaterial could be termed a second instance of the layer of material,and, similarly, a second first instance of the layer of material couldbe termed a first instance of the layer of material, without changingthe meaning of the description, so long as all occurrences of the “firstinstance of the layer of material” are renamed consistently and alloccurrences of the “second instance of the layer of material” arerenamed consistently. The first instance of the layer of material andthe second first instance of the layer of material are both instances ofthe layer of material, but they are not the same instance of the layerof material.

The terminology used herein is for the purpose of describing particularimplementations only and is not intended to be limiting of the claims.As used in the description of the implementations and the appendedclaims, the singular forms “a,” “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “comprises” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

As used herein, the term “if” may be construed to mean “when” or “upon”or “in response to determining” or “in accordance with a determination”or “in response to detecting,” that a stated condition precedent istrue, depending on the context. Similarly, the phrase “if it isdetermined [that a stated condition precedent is true]” or “if [a statedcondition precedent is true]” or “when [a stated condition precedent istrue]” may be construed to mean “upon determining” or “upon adetermination that” or “in response to determining” or “in accordancewith a determination” or “upon detecting” or “in response to detecting”that the stated condition precedent is true, depending on the context.

What is claimed is:
 1. A method of fabricating a composite conductivefilm, comprising: providing, as a matrix, a layer of cross-linkablepolymer, wherein the cross-linkable polymer is in a non-cross-linkedstate; introducing a plurality of inorganic nanowires upon a surface ofthe layer of cross-linkable polymer, wherein the inorganic nanowiresare, in isolated form, characterized by a first conductivity stabilitytemperature; embedding at least some of the plurality of inorganicnanowires into the layer of cross-linkable polymer to form an inorganicmesh within the layer of cross-linkable polymer, thereby forming thecomposite conductive film; and cross-linking the cross-linkable polymerwithin at least a surface portion of the composite conductive film,wherein following the cross-linking, the cross-linkable polymer withinat least the surface portion of the composite conductive film is in across-linked state; wherein cross-linking the cross-linkable polymerwithin at least the surface portion of the composite conductive filmresults in at least the surface portion of the composite conductive filmhaving a second conductivity stability temperature that is greater thanthe first conductivity stability temperature.
 2. The method of claim 1,wherein the plurality of nanowires has an average aspect ratio between10-1000.
 3. The method of claim 1, wherein the layer of cross-linkablepolymer comprises one of an acrylated polyurethane or an epoxy-basedalkyl-amine.
 4. The method of claim 1, wherein providing, as the matrix,the layer of cross-linkable polymer includes depositing the layer ofcross-linkable polymer on a substrate.
 5. The method of claim 4, whereinthe substrate is one of a polyester substrate or a borosilicate glasssubstrate.
 6. The method of claim 1, wherein the layer of cross-linkablepolymer comprises a polymer that undergoes cross-linking when exposed toone of ultraviolet (UV) radiation, heat treatment, or a chemicalcatalyst.